Among knock-type writing instruments, one type in which a pen point thrusts out upon depressing a knock rod at the rear end of the holder and draws back upon depressing the knock rod again, is called a double knock-type writing instrument. Further, some knock-type writing instruments comprise a safety mechanism such that when cloth or paper is held by a clip, a nub of the clip is disengaged causing the pen point to draw back.
Writing instruments of several double-knock types have heretofore been known being equipped with the safety mechanism. Here, the safety mechanism means is a mechanism which when a tip of the clip is lifted up in a state where the pen point is at the writing position protruding beyond the end of the holder, the pen point draws back into the holding position in the holder. By this mechanism, clothing will not be stained in case the writing instrument is inserted in, for example, a chest pocket of clothing with the pen point extended.
Japanese Patent No. 3681203 discloses a double knock-type writing instrument of the cam knock system, wherein when a nub of a clip engages with a protuberance of a rotor causing the rotor to stop rotating at a writing position, the writing can be effected. It discloses a safety mechanism, wherein when the tip of the clip is lifted up causing the nub to be disengaged, the rotor draws back to a holding position while being rotated.
Japanese Patent No. 3327879 discloses a system in which when a protuberance of a clip nub comes in collision with a protuberance of a rotor, the rotor stops rotating at a writing position enabling the writing to be effected.
However, the systems disclosed by the above two documents are accompanied by problems as described below. That is, the sum of diameter of a refill (ink container), thickness of a knock member and thickness of the holder becomes the outer diameter of the holder resulting in an increase in the diameter of the holder.
Further, because of such a relationship that the knock member (rotor) engages with the clip nub, it becomes necessary to attain an engaging position. There, a problem remains in that the knocking member must be to be long causing the length of the refill to be shortened correspondingly, the amount of the ink to be decreased and the writing distance to be shortened.
A further problem is that, in order to cope with the above problem, the knock member is extended toward the end of the holder up to a position of a hole with which the nub of the clip engages, and the refill is buried in the knock member. Therefore, the remaining amount of the ink cannot be seen from the outer side.
Japanese Patent No. 3730574 discloses a system in which a moving element that engages with the rotor is brought into engagement with a clip nub due to a protuberance on the side opposite to the engaging portion.
However, the system of this document, is also accompanied by a problem in that, if the knocking operation is effected under such conditions that the safety mechanism has already functioned, the pen point thrusts out, but simply draws back and is not set to the writing position.